The invention relates to an AC-DC converter comprising
input terminals which are to be connected to the poles of a supply voltage source supplying an alternating voltage and output terminals,
rectifier means coupled to the input terminals for rectifying the alternating voltage,
an inductive element coupled to the rectifier means,
a buffer capacitance coupled to the output terminals,
a unidirectional element coupled between the inductive element and the buffer capacitance,
a switching element coupled to the inductive element for controlling a current through the inductive element,
a control circuit coupled to a control electrode of the switching element for generating a periodic control signal for rendering the switching element alternately conducting and non-conducting at a frequency f, and provided with
a first circuit part for generating a first signal which is a measure of the instantaneous amplitude of the current in the inductive element, and
a second circuit part for generating a second signal which is directly proportional to the instantaneous value of the amplitude of the alternating voltage,
a comparator a first input of which is coupled to an output of the first circuit part, a second input of which is coupled to an output of the second circuit part, and an output of which is coupled to the control electrode of the switching element.
Such an AC-DC converter is disclosed in U.S. Pat. No. 4,683,529. The control circuit of the known AC-DC converter renders the switching element conducting during a first time interval t-on, which is substantially constant during each half period of the alternating voltage supplied by the supply voltage source. During the first time interval t-on, the current in the inductive element increases substantially linearly. The value of t-on corresponds to the power taken at the output terminals. As the value of t-on is substantially constant during each half period of the alternating voltage, the value of the current taken from the supply voltage source, averaged over a period of the control signal, is substantially proportional to the instantaneous amplitude of the alternating voltage. It is thus achieved that the power factor of the known AC-DC converter is comparatively high. During the second time interval t-off, the current in the inductive element decreases substantially linearly. In the known AC-DC converter, the control circuit renders the switching element conducting again almost immediately after the current in the inductive element has become substantially equal to zero. This control of the switching element is referred to as xe2x80x9ctransition modexe2x80x9d. As the current in the inductive element is substantially zero, the same applies to the current through the unidirectional element. It is thus achieved that, when the switching element becomes conducting, only a comparatively small power dissipation occurs in the unidirectional element. The frequency of the control signal is often chosen to be comparatively high because this enables both the inductive element and EMI filters, which are often arranged between the input terminals and the rectifier means, to be chosen so as to be comparatively small. As a result, the AC-DC converter is comparatively small and inexpensive. However, if the power taken at the output terminals decreases, or if the amplitude of the alternating voltage supplied by the voltage supply source increases, the value of t-on is reduced by the control circuit. Also at such a low value of the power taken or at a comparatively high value of the amplitude of the alternating voltage, the known AC-DC converter operates in the transition mode, as a result of which the frequency of the control signal increases. A drawback of the known AC-DC converter resides in that, at a high frequency, the majority of the known control circuits are insufficiently capable of sufficiently accurately controlling the time interval t-on, so that instabilities in the operation of the AC-DC converter may occur. The quantity of power dissipated in the switching element also is comparatively high at a comparatively high frequency of the control signal.
It is an object of the invention to provide an AC-DC converter which can operate in a stable manner over a large range of the power taken and over a large range of the amplitude of the alternating voltage supplied by the supply voltage source, and which has a high power factor, a low THD and a low power dissipation in the components.
To achieve this, an AC-DC converter of the type mentioned in the opening paragraph is characterized in accordance with the invention in that the control circuit additionally comprises a third circuit part for generating an offset signal, and an adder circuit part for combining the first signal and the offset signal, an output of which is coupled to the first input of the comparator.
It has been found that an AC-DC converter in accordance with the invention can be used in a comparatively large range of the amplitude of the alternating voltage and in a comparatively large range of the power taken at the output terminals. Within these two ranges, the power factor of the AC-DC converter is comparatively high and the THD is comparatively low.
Good results have been obtained using an AC-DC converter in accordance with the invention wherein the offset signal has a constant amplitude. If the offset signal has a constant amplitude, then the AC-DC converter does not take power from the voltage-supply source in the vicinity of the zero-crossings of the alternating voltage. A constant amplitude for the offset signal can be achieved in a comparatively simple manner in that the third circuit part comprises an ohmic resistance.
Good results have also been achieved with embodiments of an AC-DC converter in accordance with the invention, wherein the offset signal is a periodic signal whose frequency is equal to the frequency of the rectified alternating voltage. More particularly, good results are achieved, particularly when the power taken at the output terminals is comparatively small, if the amplitude of the offset signal is at a local minimum when the amplitude of the rectified alternating voltage is maximal. Such a form of the offset signal can be achieved in a comparatively simple and very reliable manner in that the inductive element comprises an auxiliary winding, and the third circuit part is coupled to said auxiliary winding and provided with
a series arrangement of a diode and two impedances, and
a capacitive element coupled to a junction point of the two impedances.
The impedances preferably comprise ohmic resistances.
In a further embodiment of an AC-DC converter in accordance with the invention, the third circuit part is coupled to an output of the control circuit, and the third circuit part is provided with a series arrangement of two impedances and with a capacitive element coupled to a junction point of both impedances. Good results have also been achieved by using this embodiment. The impedances preferably comprise ohmic resistances.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.